The PSCOF method was recently invented and used at Kent State University to prepare adjacent parallel layers of polymer and liquid crystal. This method is disclosed in U.S. Pat. No. 5,949,508, and is incorporated herein by reference. The PSCOF method involves a process similar to that used in the fabrication of polymer dispersed liquid crystal films. The polymer and liquid crystals are mixed in a predetermined proportion and placed between two substrates with well defined thickness or over one substrate. When a beam of UV light is made incident from one side, the phase separation process is initiated. The rate of polymerization (and phase separation) is faster near the source of UV radiation due to higher intensity. As the phase separation starts, the organic or liquid crystal material is expelled from the polymerizing portion and begins to migrate away from the source of UV radiation. As a result, a uniform film of polymer is obtained on one side of the cell, and a substantially uniform layer of liquid crystal is formed on the opposite side of the cell away from the UV light source. The separation of liquid crystal from polymer can be aided by a pre-disposed alignment layer which the liquid crystal material likes to wet, on the substrate away from the UV source. As such, an aligned liquid crystal film may be formed.
The alignment layer is typically made up of a long chain polymeric material which is traditionally later subjected to processes such as mechanical rubbing or ultraviolet exposure to alter the surface properties. There are several generally accepted techniques for forming an alignment layer on the substrate of a liquid crystal device. Commonly used methods are rubbing or photo-alignment of organic/polymer films, and evaporation of inorganic materials. Although each method is capable of aligning the liquid crystal material, each method has particular drawbacks.
The most commercially used method of forming an alignment layer is the rubbing technique. In this method, for example, a polyamic acid is spin-coated or otherwise deposited on a substrate. The polyamic acid is subjected to two heat treatments, soft-bake and hard-bake, to form a polyimide (PI) film. After an appropriate cooling period, the PI film is rubbed by a cloth, such as velvet, in a uniform singular direction. When the liquid crystal later comes in contact with the rubbed surface, the liquid crystal aligns along the rubbing direction. Unfortunately, this method can cause mechanical damage and generate electrostatic charges, both of which adversely affect liquid crystal displays, especially those that employ thin-film transistors. The rubbing method also generates dust from the cloth and PI, which may contaminate the liquid crystal material.
Another method of forming an alignment layer involves forming a PI film on a substrate, as described above. A linearly polarized ultraviolet light is projected onto the surface of the PI film to produce the desired molecular alignment. The UV radiation anisotropically photodissociates photosensitive bonds in the PI, including those in the imide ring. This selectively reduces the polarizability of PI molecules and changes the surface properties and morphology. Unfortunately, this method results in alignment layers with weak anchoring of liquid crystals and poor thermal and chemical stability. Furthermore, this method requires costly multi-step processing. Yet another drawback of this method is that it only provides a limited charge holding ratio and less thermal stability when compared to the rubbing technique.
A similar method of preparing an alignment layer by using photo-sensitive polymers is also known. For example, photo-sensitive polymers such as poly(vinyl)4-methoxycinnamate (PVMC); poly(vinyl)cinnamate (PVC); and polysiloxanecinnamate films may be used to align liquid crystal material. These materials, when exposed to a linearly polarized ultraviolet light (LPUV), initiate a photo-reaction after evaporation of the solvent. This method causes cross-linking (bonding) and resultant orientation of the side chain molecules uniaxially in a direction determined by the direction of linear polarization. However, this process does not chemically fix the orientation of the molecules and the alignment is lost upon exposure to normally occurring un-polarized UV light. Moreover, the chemical composition of the materials is lost over time. Consequently, the alignment layer thus produced does not provide a fixed, stable orientation of liquid crystal material.
Yet another method for forming an alignment layer on a substrate is deposition by evaporation of inorganic materials onto the surface of the substrate at various incidence angles. This forms an alignment layer which physically orients the director of the liquid crystal. Inorganic materials which have been used include silicon oxides and magnesium oxides. This deposition method has proven to be cumbersome and difficult to use in a manufacturing process.
Another process for forming alignment layers, developed by Kent State University, is the in-situ UV exposure method. This method is disclosed in U.S. Pat. No. 5,936,691, and is incorporated herein by reference. The in-situ method is similar to the conventional process of exposing PI film to polarized UV light. The in-situ method, however, exposes the polyimide film (PI) to UV radiation while the film is being soft- and hard-baked. The resulting alignment layers have higher anchoring energies and are thermally more stable compared to the conventional UV exposure technique. For example, cells prepared with the conventional method lose alignment when maintained at 100° C. for 12 hours, while cells prepared by the in-situ method, show no sign of deterioration at 300° C. for 12 hours. The in-situ method of forming an alignment layer avoids many of the drawbacks of other methods known in the art, while requiring fewer and simpler processing steps.
Although the in-situ method has been shown to be effective, it still requires that the alignment film must still be properly disposed and processed. Moreover, for all the alignment methods discussed above, the alignment layer may be damaged if care is not taken in transferring the substrate between the manufacturing stations. It is also theorized that currently known alignment layers only directly affect the liquid crystal material adjacent thereto.
It has also been disclosed in U.S. Pat. No. 6,083,575 that a polymer dispersion type liquid crystal (PDLC) element may be manufactured by applying laser interference light to a polymerizable composition containing a polymerizable compound having a photo-dimerizable structure and low molecular weight liquid crystals. The laser interference light causes polymer phase separation. Polarized light is then applied to orient the low molecular liquid crystal. As stated in that patent, a polymer dispersion type liquid crystal is a display element in which liquid crystals are dispersed in the interstices of polymers of a three dimensional structure. However, while U.S. Pat. No. 6,083,575 teaches the use of a photo-sensitive polymer, the teachings of this patent are limited to polymer dispersion type liquid crystal cells. Various limitations are inherent in PDLC cells however, compared to cells using phase separated composite organic films for example. PDLC cells have higher voltage requirements, slower switching speeds and very low multiplexability. Phase separated composite organic films also allow for the formation and use of more-well defined geometries in liquid crystal displays, especially in bulk form. No teaching or suggestion is made regarding the construction of a liquid crystal element of a type other than a polymer dispersion type liquid crystal, such as a phase-separated composite organic film as present in the present application or the use of photo-sensitive polymers to create an alignment film.
In light of the foregoing, it is evident that there is a need in the art for a light modulating device comprising a phase separated composite organic film, and a method for manufacturing such a film, that is homogeneously aligned during formation, thus obviating the need for the preparation and processing of a separate and distinct alignment layer.